Endoscopes often have light guides of various diameters that may be selectively attached to a single lighting system. Thus, it is desirable to have a lighting system for light guides be capable of efficiently illuminating light guides that cover a range of diameters from narrow to wide. Further, it is desirable for such a lighting system to be capable of inputting a large quantity of light to the attached light guide so as to provide a bright illumination for the endoscope.
An arc-discharge type lamp, such as a xenon lamp or a metal halide lamp, is generally used as a light source for inputting illuminating light to a light guide of an endoscope. Such a lamp is formed by sealing two electrodes (an anode and a cathode) as well as an arc-discharge gas within a transparent enclosure, such as a heat-resistant glass material. The light emission region between the two electrodes has a light emission area of spatially finite size. In order to gather light that is radiated from the arc-discharge area in various directions onto an end of the light guide, a reflector is generally arranged so as to partially surround the electrodes. Moreover, the reflector may be provided as an integral component of the lamp and thus serves as one section of the transparent cover of the lamp. An arrangement wherein the reflector reflects light emitted by an arc-discharge directly onto the end of the light guide may provide a sufficient illumination for an endoscope when a light guide of large diameter is used with the endoscope.
When a light guide having a "narrow" (herein defined as 3 mm or less) diameter is to be used with an endoscope, the lighting system for light guides in such a case conventionally arranges a condenser lens between the lamp and the end of the light guide in order to condense the light reflected by the reflector into a smaller region, thereby enabling the entire region of condensed light to be incident onto the narrow end of the light guide. However, it has been found that there is a remarkable deterioration over the life of the lamp in the amount of light that is input to an end of a light guide in the case where the outer diameter of the light guide is 3 mm or less.
FIG. 7 illustrates the amount of light incident on an end of a light guide of narrow diameter as a function of time for an arc-discharge lamp source as the lamp ages. As illustrated in the figure, the quantity of incident light on the light guide end decreases rapidly at first, and then decreases more gradually with increasing age of the lamp. What is believed to be the primary reason for this decrease will now be explained with reference to FIG. 6.
FIG. 6 is a graph which shows the brightness of light emitted by an arc-discharge lamp as a function of position along a line between the two electrodes for a direct current, arc-discharge type lamp. The shape and position of the anode 11 and cathode 12 are illustrated below abscissa of the graph. As is apparent from FIG. 6 a luminescent region having the highest intensity (marked by slanted lines in the figure) occurs adjacent the portion of the cathode 12 that is nearest the anode. This luminescent region will herein be termed the "hot spot", and the portion of the cathode nearest the anode will herein be termed the "tip" of the cathode. It is believed that an important reason for the decrease in light intensity with time (as illustrated in FIG. 7) is due to the depletion of the cathode 12 as the lamp ages. Thus, the tip of the cathode as well as the hot spot adjacent the tip of the cathode shift to the right in FIG. 6 as atoms of the cathode are torn away by action of the flow of electrons from the cathode to the anode during operation of the lamp.
Another reason for the decrease in light intensity with time is that the atoms that are torn away from the cathode by the arc-discharge current are then deposited over time onto the inner surface of the glass envelope of the light source and, in the case where a reflector is made integral with the light source and within the glass envelope, then are deposited on the reflector surfaces as well. These soot-like deposits build up on the reflector and on the interior surface of the glass envelope and increasingly block more light from being incident on the light guide end as the lamp ages. Also, the spacing between the cathode and the anode tends to increase as the cathode is depleted and this can reduce the luminosity of the arc discharge as the lamp ages.
Whereas a change in position of the hot spot resulting from depletion of the cathode has only a small affect on the amount of light coupled into a light guide of large diameter, a similar change in position of the hot spot in the case where the light guide is of narrow diameter (3 mm or less) has been found to cause a remarkable degradation in the quantity of light in the light guide. Prior to the present invention, variation in quantity of light input to an end of a light guide has been especially troublesome for light guides having an outer diameter 3 mm or less as a lamp in the lighting system ages. Of course, any significant variation in light intensity in a lighting system of an endoscope is undesirable.